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North UK

carbs haven’t been tuned and it’s running lean, (I don’t know how long it’s been like this)

A good idea to give them a full clean session,  and perharps consider improving the mid-range with this tutorial  and this one too for main jet sizing guide.
Do not touch the “pilot mixture” screws, its the abyss you may spend weeks sorting it out if unsure what to do, the Korean’s set the mixture just fine.   They just put smaller jetting for euro3 stuff (already lean from factory)

Any can change or any pipe change =  Jetting mandated,  save for the pistons from glazing up, if bike gets too hot in summer it may not help the rear motor which is already hotter than front, hence it gets a bigger jet than front  (eg. 90 Front  / 92.5 rear)
You want the spark plugs nice and dark brown in the middle & without all the threads burnt up black


Bought a new project 2016 gtr125 and it comes with a sp engineering slip on ……
will changing to a stock exhaust for the bike make it run better again?

Stick a baffle inside the aftermarket can,  and if possible  make “the baffle hole” about the size of 10p coin (crush, tap, hammer etc, get creative)

When the bike’s gasses escape from a smaller hole the size of that coin, it will pull better low down 100% , top end will still be there too.   It doesn’t need a huge hole or it screams away but moves a little slower than before.   You can notice this yourself next time you can ride the bike.

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